Process for the production of large single crystals of boron phosphide



United States Patent Oflice 3,009,780 Patented Nov. 21, 1961 3 009,780PROCEQS FOR THE PRODUC1ION F LARGE SINGLE CRYSTALS 0F BORON PHOSPHIDEBobbie D. Stone, Miamisburg, Ohio, or to Monsanto Chemical Company, St.Louis, Mo., a eorpora- (ion of Delaware No Drawing. Filed June 22, 1959,Ser. No. 821,632

9 Claims. (Cl. 23-204) The present invention relates to a new method forthe production of a large single crystal form of boron phosphide. It isan object of this invention to provide a new and economical method forthe production of boron phosphide characterized as having a cubiccrystalline structure and existing as well defined elongated singlecrystals which are larger than those obtained in conventional methods.It is a further object to provide a method for the production of singlecrystals of boron phosphide from elemental phosphorus and phosphoruscompounds including phosphides which are reacted with metal-boroncompounds or elemental boron in solution in a liquid metal. Furtherobjects and advantages of my invention will be apparent from thefollowing description.

The present process for the production of crystalline boron phosphide ina single crystal form is based upon a chemical reaction which occursunder specified conditions within the molten metal matrix composed of asingle metal or an alloy. A phosphorus source such as elementalphosphorus, a phosphorus alloy or a metal phosphide reacts with theboron source such as elemental boron, a boron alloy or a metal boridewhich is dissolved in this medium or matrix. In the present description,the term "alloy" refers to all combinations of metals and alsometalloids including those with boron or phosphorus. According to thisdefinition a chemical compound, and also compositions ofnon-stoichiometric proportions are included in the term alloy. Themetal-phosphorus alloys which are contemplated in the present inventionare those of copper, aluminum, gallium, indium, silicon, titanium,zirconium, germanium, chromium, manganese, iron, cobalt, nickel,ruthenium, rhodium, palladium, osmium, iridium and platinum. The boronsource for the present reaction is a boron alloy of copper, aluminum,gallium, indium, silicon, titanium, zirconium, germanium, chromium,manganese, iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium,iridium, platinum and carbon used singly or in combination, for exampleas an iron-nickel-boron alloy.

The matrix in which the boron source and the phosphorus source react isa single metal or mul-ti-component metal alloy selected from the groupconsisting of copper, aluminum, gallium, indium, silicon, titanium,zirconium, germanium, chromium, manganese, iron, cobalt, nickel,ruthenium, rhodium, palladium, osmium, iridium and platinum. In carryingout the growth of single crystals, the components are heated to atemperature above the freezing point and below 1800 C., so that themetal or alloy selected must melt below '1 800 C.

The term single crystal as employed in the present invention refers tocrystalline material in which the said single crystals have grossphysical dimensions such that at least one dimension of the crystallineproduct is at least 0.1 millimeter.

It has been found that a critical limitation which must be observed ifsingle crystalforms of boron phosphide are to be obtained, is that themolten metal matrix containing the dissolved boron source and thedissolved phosphorussourcemustbecooledataratevaryingwithin the range offrom 1' C. to 300 C. per hour from the molten condition to the freezingpoint, a generally preferred range being 3 C. to 60 C. per hour. Aspecific preferred cooling rate with regard to iron as the matrix isfrom 5 C. to 60 C. per hour, a preferred cooling rate in the use ofmetallic copper as the molten matrix is from 3 C. to 60 C. per hour anda prefered rate of cooling with nickel as the metallic matrix is from 3C. to 60' C. per hour.

It has also been found that a critical limitation in the production of asingle crystal form of boron phosphide is the control of the boroncontent dissolved in the molten metal matrix. It has been found that aconcentration range broadly from 0.1% to 50% by weight of boron, isessential in order successfully to produce the single crystal type.Preferred ranges of boron concentration with respect to the use ofdifferent metals as the matrix are set forth in the table below. In thistable the percent of boron by weight in the matrix is shown both withrespect to a broad range and also a narrow or preferred range for therespective metal matrices.

Whenacombinationofmetalsisusedasthemauix, the weight average boronranges in accordance with relative proportions are applicable.

It has been found that the use of concentrations of boron other than inthe critical range set forth above either gives no boron phosphide oryields products which are not single crystals. It is also preferred todissolve the boron component completely in the molten matrix beforeadding the phosphorus component. However the boron or the phosphorussources may be added simultaneously or in this sequence in the instantprocess.

In conducting the present process, the molten matrix, containing boronand phosphorus is subjected to cooling. A study of the cooling curveshows that a temperature is reached at which the amount of boron andphosphorus in solution are equal to the saturation value at suchtemperature. As the solution is cooled further, boron phos phideprecipitates. However, for the production of large single crystals, thepresent controlled rate of cooling is necessary.

In a preferred embodiment of the invention, the process begins with ametal boride, which is dissolved in the desired metal matrix, such metalmatrix being either the same as that of the starting boride or adifierent metal of the group defined above. For example, in employingnickel boride as the source of the boron, the metal matrix may be any ofthe preferred group consisting of copper, aluminum, gallium, indium,silicon, titanium zirconium, germanium, chromium, manganese, iron,cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium andplatinum. When the working matrix has thus been obtained with the boronpresent in the desired proportions as set forth herein, the phosphorusmay be introduced directly as phosphorus vapor which is passed intocontact with the molten metal solution. However, it is also contemplatedthat a phosphorus source such as a metal phosphide or alloy may be addeddirectly. It is found that when the phosphorus source is thus dissolvedin the matrix, containing the boron, and the entire system cooled to thefreezing point at the specified rate, the desired single crystal form ofboron phosphide is precipitated. To prevent excessive thermal shock tothe boron phosphide crystals in the matrix it is advantageous to coolthe solidified ingot slowly to room temperature. However, this coolingrate from the freezing point to room temperature need not nwessarily bewithin that specified for cooling the melt to the freezing point.

In carrying out this process, certain modifications corning within thebroad scope of the invention may be used. In one embodiment the entirecharge is heated to the high temperature desired, e.g., above themelting point, but below 1800 C. and then cooled at the rates describedabove. This procedure may be modified further by placing the charge inan area with a temperature gradient, i.e., having one end hotter thanthe other, and cooling slowly.

Another embodiment is the application of the present method to zonemelting in which the body of the metalboron alloy is reacted in anatmosphere of phosphorus vapor. This is conducted by slowly pulling thecharge through a very narrow induction-heating coil so that only a smallcross section is molten at one time. Boron and phosphorus then react inthe molten none, and as the reaction mixture leaves the hot zone, itcools at the rate described above. This gives rise to large crystals.Since fewer crystal nuclei are formed when only a small zone is moltenat one time, this process gives even larger crys tals than in the casewhere the entire charge is molten at once.

In all of the above methods, after the molten matrix containing thesingle crystals of boron phosphide has completely solidified, the boronphosphide is obtained by dissolving away the metal with a reagent whichdoes not dissolve crystalline boron phosphide. Examples of such reagentsare hydrochloric acid, nitric acid, sulfuric acid and aqua regia.

The following examples illustrate specific embodiments of thisinvention:

Example 1 Nickel powder 100 mesh) and boron powder (-325 mesh) werethoroughly mixed in proportions to give a mixture containing 93% Ni and7% B by weight. A charge of 20.80 g. of this mixture was loaded in agraphite boat and the boat placed in a mullite tube closed on one endand sealed to Pyrex glass on the other. The tube was evacuated to apressure of 10- mm. Hg and then sealed from the vacuum system. Afterbreaking a Pyrex breakseal leading to a phosphorus reservoir, about g.white phosphorus was distilled into the ceramic tube. The Pyrex end ofthe tube was sealed and the tube placed in two adjoining furnaces insuch a manner that the boat containing the nickel-boron mixture was inthe center of a large Globar tube furnace, while the Pyrex end of thetube was enclosed in a small Nichrome resistance-wound furnace. TheGlobar furnace was heated to 1400" C. and then the small furnace washeated rapidly to 280 C. and gradually raised to 410 C. At thistemperature, the vapor pressure of phosphorus inside the tube was aboutone atmosphere. The temperature of the Globar furnace was then loweredat the uniform rate of 5/hour until it reached the solidification point,1170 C. Both furnaces were turned off and allowed to cool to roomtemperature. After cooling, the tube was opened by breaking the Pyrexsection and the ingot containing crystalline boron phosphide in thenickel phosphide matrix Example 2 The experiment was carried out in thesame manner as Example 1 except that a mixture containing 96% Ni and 4%B was used. A charge weighing 16.00 g. was

placed in the ceramic tube as described above and about 15 g. whitephosphorous distilled in. After heating to 1400 C. and heating thephosphorus reservoir to 410 C., the charge was cooled at a uniform rateof 5 C./hr. Upon leaching away nickel phosphide with hot dilute nitricacid, the product was obtained as transparent crystals.

Example 3 A charge of 96% Ni4% B mixture was placed in a graphite boatin a mullite tube. The tube was evacuated and phosphorus distilled in asin Example 1. After heating the Globar furnace to 1400" C. and raisingthe phosphorus reservoir temperature to 410 C., the charge was cooled ata uniform rate of 37 /hour until the freezing point was reached. Uponcooling and leaching the ingot, the product was obtained as transparentred crystals, some of which were 5 mm. in one dimension.

Example 4 A 10.97 g. charge of 92.5% Fe-7.5% B mixture was placed in agraphite boat in a mullite tube. After evacuating, white phosphorus wasdistilled into the tube which was then sealed. The tube was placed in aGlobar furnace with an auxiliary furnace around the phosphorusreservoir. The charge was heated to 1400 C. and the phosphorus reservoirheated to 410 C. to drive the phosphorus into contact with theboron-containing melt. The charge was cooled at a uniform rate of 6 perhour to the freezing point at 1260 C. The furnaces were turned off andallowed to cool to room temperature. The ingot was removed from the tubeand iron phosphide was dissolved away with hot concentrated aqua regia.Crystalline boron phosphide was left behind as transparent crystals.

Example 5 This experiment was carried out at atmospheric pressure usinga continuous flow gas system. A 32.44 g. charge of a 92.5% Fe-7.5% Bmixture was placed in a graphite boat in an open-ended mullite tubeconnected in such a manner that gas could be passed through the tube.The tube was placed in a Globar furnace with the charge located in thecenter of the furnace and with a thermocouple reading the temperature atthe location of the charge. A graphite boat containing red phosphoruswas placed in the tube at one end protruding from the Globar furnace andthe tube was surrounded with an auxiliary Nichrome furnace at the pointoccupied by the phosphorus boat. The Globar furnace was heated to 1400'C. and the phosphorus boat heated to 450 C. and angon was passed throughthe tube at 50 mL/min. so that the gas stream first passed over thephosphorus boat and then over the charge. The charge was then cooled ata uniform rate of about 5 lhour until a temperature of 1300 C. wasreached at which point the phosphorus furnace was turned oif and thecharge cooled to room temperature under a stream of argon. Afterleaching away the iron phosphide matrix with hot concentrated aquaregia, crystalline boron phosphide was left behind in the form oftransparent red crystals, some of which were 2-3 mm. in one dimension.

Example 6 The method of Example 5 was repeated with the modificationthat a 16.88 gm. sample of a 93% Ni-7% B alloy was placed in arefractory boat, and placed in a mullite reaction tube. The Globarfurnace surrounding the charge was heated to 1400 C. and the phosphorusfurnace heated to 425 C. A stream of argon was passed through thetubular reactor at the rate of 20 ml./min., so that the partial pressureof phosphorus vapor was only about one-tenth as much as in the precedingexample.

While the argon-phosphorus vapor stream was passed over the melt, thetemperature was lowered at a uniform rate of about 6 C. per hour until atemperature of 1230 C. was reached, slightly above the solidificationpoint. After cooling the reaction mass to room temperature, the nickelphosphide matrix containing some unreacted nickel was dissolved awaywith hot diluted nitric acid. The product left after the acid treatmentconsisted predominantly of black crystals of about 2 to 3 mm. length.This product has less phosphorus than cubic crystalline 'BP, and can becontrolled to have a formula ranging from B P to 3 F. It was found tocrystallize in the monoclinic system.

I claim:

1. Process for the production of single crystals of boron phosphidewhich comprises dissolving a boron source selected from the groupconsisting of elemental boron and boron alloys in a molten metal matrixselected from the group consisting of at least one of the metals of thegroup consisting of copper, aluminum, gallium, indium, silicon,titanium, zirconium, germanium, chromium, manganese, iron, cobalt,nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum, theproportion of boron calculated as elemental boron in the matrix being inthe range of from 0.1% to 50% by weight, and after the said boroncomponent is completely dissolved, introducing into the said mixture aphosphorus source selected from the group consisting of elementalphosphorus, and metal phosphides in suflicient proportion to combinewith the boron which is present, and thereafter cooling the said moltenmixture at a rate within the range of from 1 C. to 300 C. per hour fromthe molten condition to the freezing point.

2. Process for the production of single crystals of boron phosphidewhich comprises dissolving a boron source selected from the groupconsisting of elemental boron and metal borides in a molten metal matrixselected from the group consisting of at least one of the metals of thegroup consisting of copper, aluminum, gallium, indium, silicon,titanium, zirconium, germanium, chromium, manganese, iron, cobalt,nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum, theproportion of boron calculated as elemental boron in the matrix being inthe range of from 0.1% to 50% by weight, and after the said boroncomponent is completely dissolved, introducing into the said mixture aphosphorus source selected from the group consisting of elementalphosphorus, and metal phosphides in sufiicient proportion to combinewith the boron which is present, and thereafter cooling the said moltenmixture at a rate within the range of from 3 C. to 60 C. per hour fromthe molten condition to the freezing point, and thereafter isolating thesaid crystal form of boron phosphide from the metallic matrix.

3. Process for the production of single crystals of boron phosphidewhich comprises dissolving a boron source selected from the groupconsisting of elemental boron and metal borides in a molten metal matrixselected from the group consisting of at least one of the metals of thegroup consisting of copper, aluminum, gallium, indium, silicon,titanium, zirconium, germanium, chromium, manganese, iron, cobalt,nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum, theproportion of boron in the matrix being in the range of from 0.1% to 50%by weight, after the said boron component is completely dissolved,introducing into the said mixture a phosphorus source selected from thegroup consisting of elemental phosphorus, and metal phosphide insuflicient proportion to combine with the boron which is present, andthereafter cooling the said molten mixture at a rate within the range offrom 3 C. to 60 C. per hour from the molten condition to the freezingpoint, and thereafter isolating the said crystal form of boron phosphidefrom the metallic matrix, by dissolving the said metal in a mineral acidto leave the said crystalline boron phosphide undissolved.

4. Process for the production of single crystals of boron phosphidewhich comprises dissolving elemental boron in a molten matrix of nickelmetal, the proportion of boron being in the range of from 0.1% to 50% byweight, and after the boron is completely dissolved, introducing intothe said mixture elemental phosphorus in sufiicient proportion tocombine with the boron, and thereafter cooling the said molten mixtureat a rate within the range of from 1 C. to 300 C. per hour from themolten condition to the freezing point.

5. Process for the production of single crystals of boron phosphidewhich comprises dissolving elemental boron in a molten matrix of nickelmetal, the proportion of boron being in the range of from 0.1% to 50% byweight, and after the boron is completely dissolved, introducing intothe said mixture elemental phosphorus in sufiicient proportion tocombine with the boron, and thereafter cooling the said molten mixtureat a rate within the range of from 1 C. to 300 C. per hour from themolten condition to the freezing point, and thereafter isolating thesaid crystal form of boron phosphide from the metallic matrix, bydissolving the said metal in a mineral acid to leave the saidcrystalline boron phosphide undissolved.

6. Process for the production of single crystals of boron phosphidewhich comprises dissolving elemental boron with metallic iron, theproportion of boron in the matrix being in the range of from 0.1% to 50%by weight, after the said boron is completely dissolved, introducinginto the said mixture elemental phosphorus in sufficient proportion tocombine with the boron which is present, and thereafter cooling the saidmolten mixture at a rate within the range of from 1 C. to 300 C. perhour from the molten condition to the freezing point.

7. Process for the production of single crystals of boron phosphidewhich comprises dissolving elemental boron with metallic iron, theproportion of boron in the matrix being in the range of from 0.1% to 50%by weight, after the said boron is completely dissolved, introducinginto the said mixture elemental phosphorus in sufficient proportion tocombine with the boron which is present, and thereafter cooling the saidmolten mixture at a rate within the range of from 1 C. to 300 C. perhour from the molten condition to the freezing point, and thereafterisolating the said crystal form of boron phosphide from the metallicmatrix by dissolving the said metal in a mineral acid to leave the saidcrystalline boron phosphide undissolved.

8. Process for the production of single crystals of boron phosphidewhich comprises dissolving elemental boron with metallic copper, theproportion of boron in the matrix being in the range of from 0.1% to 50%by weight, after the said boron is completely dissolved, introducinginto the said mixture elemental phosphorus in sufiicient proportion tocombine with the boron which is present, and thereafter cooling the saidmolten mixture at a rate within the range of from 1 C. to 300 C. perhour from the molten condition to the freezing point.

9. Process for the production of single crystals of boron phosphidewhich comprises dissolving elemental boron with metallic copper, theproportion of boron in the matrix being in the range of from 0.1 to 50%by weight, after the said boron is completely dissolved, introducinginto the said mixture elemental phosphorus in sufficient proportion tocombine with the boron which is present, and thereafter cooling the saidmolten mixture at a rate within the range of from 1 C. to 300 C. perhour from 7 1 s I the molten cowition to the freezing point, andthereafter OTHER REFERENCES isolating the said crystal form of boronphosphide from the metallic matrix by dissolving the said metal in amin- Popper ct Nature 1075 7 eral acid to leave the said crystallineboron phosphide van Wale Phosphorus and Its C0mPounds, 1958'undissolved. 5 vol. 1, pp. 131, 146.

' Stroughton et al.: Engineering Metallurgy, 2d ed.,

References Cited in the file of this patent 1930, PP-

Hansen: Constitution of Binary Alloys, 2d ed.-, 1958,

UNITED STATES PATENTS pp. 248, 249-252, 256, 257. 2,124,509 McKenna July19, 1938 m

1. PROCESS FOR THE PRODUCTION OF SINGLE CRYSTALS OF BORON PHOSPHIDEWHICH COMPRISES DISSOLVING A BORON SOURCE SELECTED FROM THE GROUPCONSISTING OF ELEMENTAL BORON AND BORON ALLOYS IN A MOLTEN METAL MATRIXSELECTED FROM THE GROUP CONSISTING OF AT LEAST ONE OF THE METALS OF THEGROUP CONSISTING OF COPPER, ALUMINUM, GALLIUM, INDIUM, SILICON,TITANIUM, ZIRCONIUM, GERMANIUM, CHROMIUM, MANGANESE, IRON, COBALT,NICKEL, RUTHENIUM, RHODIUM, PALLADIUM, OSMIUM, IRIDIUM AND PLATINUM, THEPROPORTION OF BORON CALCULATED AS ELEMENTAL BORON IN THE MATRIX BEING INTHE RANGE OF FROM 0.1% TO 50% BY WEIGHT, AND AFTER THE SAID BORONCOMPONENT IS COMPLETELY DISSOLVED, INTRODUCING INTO THE SAID MIXTURE APHOSPHORUS SOURCE SELECTED FROM THE GROUP CONSISTING OF ELEMENTALPHOSPHORUS, AND METAL PHOSPHIDES IN SUFFICIENT PROPORTION TO COMBINEWITH THE BORON WHICH IS PRESENT, AND THEREAFTER COOLING THE SAID MOLTENMIXTURE AT A RATE WITHIN THE RANGE OF FROM 1*C. TO 300*C. PER HOUR FROMTHE MOLTEN CONDITION TO THE FREEZING POINT.